This invention relates generally to the ultrasonic welding of workpieces and more particularly to a method of ultrasonically cutting and sealing a membrane between two thermoplastic pieces. Illustratively, the membrane is a woven or non-woven filter continuously welded around its periphery between mating halves of a filter housing.
Commercially available small filters may include a pair of dished housing members joined together at their rims to form a hollow body. A central nipple in each dished housing member provides a connection to the tubing carrying the fluid to be filtered. Typically, one of the nipples is proportioned to fit into the tubing, and its housing member is designated the male housing member; the other nipple is proportioned for the tubing to fit into it, and its housing member is designated the female housing member.
A woven and/or non-woven porous filter element, in the form of a disk, is trapped between the dished housing members. In one arrangement, a woven filter disk is placed over a non-woven filter disk, with the woven filter on the male housing (inlet) side. The filter elements are held at their periphery by the rims of the housing members. For sterility and to prevent leakage, it is important that the filter disks be totally contained within the filter body, without extending beyond the periphery of the body.
Filters of this type may be assembled by ultrasonic welding if the parts are made of thermoplastic material. The filter disk is ultrasonically welded to the rim of one of the housing members to form a first assembly, and then the rim of the second housing member is ultrasonically welded to the assembly. This approach requires careful handling of the filter disks, is liable to form incomplete seals, and produces a thermoplastic material flash around the area of the weld. The flash must be trimmed in a separate operation.
To eliminate the need for precutting the filter disks and the problems of handling such disks, the filter material may be placed over the first housing member, then cut and welded ultrasonically in one operation. This approach, however, produces a groove in the first housing member. It thereby weakens the assembly and increases the chance of leakage.
Similar problems occur whenever the periphery of a membrane is ultrasonically welded between housing halves.